1. Field of the Invention
The invention relates to digital communication and, more particularly, to the coding of audio and data signals in cordless telephones.
2. Description of the Related Art
Cordless telephones can use two basic types of audio transmission: analog or digital. Digital cordless telephones offer multiple advantages over analog cordless phones, but are typically more expensive to implement. Digital coding of the audio allows a series of xe2x80x9c1xe2x80x9ds and xe2x80x9c0xe2x80x9ds to be sent over the radio part of the cordless telephone. This allows the audio information to be securely transmitted and received. Digital coding offers a higher quality transmission because analog noise that occurs in the communication link is not added into the audio information, providing a low-noise link. The penalty for digital audio is the cost of implementation. A significant amount of circuitry is required to implement the digitizing and compression of the audio.
Narrow band digital cordless telephones have historically implemented digital audio using one of two methods: ADPCM and CVSD. CVSD (continuously variable slope delta-modulation) is a simple method for digitally encoding a voice signal. Because of the simplistic way the voice is digitized, however, quality suffers in this method. At practical data rates, the quality of CVSD digital voice is not at the level of a wired telephone. ADPCM (adaptive differential pulse code modulation) was the solution to the voice quality problems of CVSD, but came at a greater implementation cost penalty. ADPCM yielded voice quality equal to that of a wired telephone; however, the ADPCM digitizing technique requires a relatively complex implementation, driving up the cost to the end user.
The other issue in a digital narrow band cordless telephone that must be considered is the data rate, which is inversely related to the energy per bit the radio uses for transmission. In a narrow band cordless telephone with a fixed transmission power, higher data rates result in a shorter range due to the correspondingly lower energy per bit. CVSD has typically been implemented at 48 kb/s. ADPCM has typically been implemented at 32 kb/s. These numbers show that ADPCM generally provides a longer range than CVSD along with the previously stated voice quality advantage, but again, with a penalty in implementation cost.
Although various designs of digital cordless telephones are available, those designs have in many ways not adequately met the consumer""s need for quality as well as economy. A digital cordless telephone that meets those expectations of consumers, thus, would provide significant improvement and advance in the technology. The consumer cordless telephone market demands lower and lower implementation costs at a higher quality level. As this happens, new trade-offs and approaches are needed.
Described herein is a cordless telephone system and method using PCM techniques for encoding an audio signal. The audio signal may be human voice, music, a modem signal, or any other analog signal in a predetermined frequency range. The PCM coding scheme provides distinct advantages over the previously used lossy coding schemes, such as ADPCM and CVSD. By using this efficiently implemented coding scheme, the system substantially reduces implementation costs, a significant consideration in cordless telephones designed for residential use and in other settings where cost is an important factor. Using a PCM encoding scheme, such as A-law or xcexc-law, provides a low cost digital telephone with excellent voice quality and a range that is acceptable for a low-cost digital cordless telephone. Further, since PCM does not degrade the audio signal quality, using it improves the quality of transmission for signals such as modem signals, another important factor for many end users.
This disclosure presents a communication system, such as a portable telephone, that has a handset and a base unit. The handset and base unit are coupled wirelessly, through an RF or IR link. The base unit receives an incoming telephone signal from a telephone connection and converts it to an incoming wireless PCM signal that is transmitted to the handset. The handset then converts incoming wireless PCM signal to an incoming audio signal that may be heard by a user. In the outgoing direction, the base unit receives an outgoing audio signal and in response generates an outgoing wireless PCM signal. The base unit receives the outgoing wireless PCM signal, converts it to an outgoing telephone signal, and provides the outgoing telephone signal to the telephone connection. The communication system may be configured to communicate with a telephone network through wired, fiber-optic, cellular, or wireless local loop links. Additionally, the links may carry analog or digital signals.
In one embodiment, the communication system is comprised in a wireless local loop system. The base unit communicates with a plurality of customer-specific portable units through wireless PCM links. The base unit is coupled to a central telephone office through an RF link.
A method is presented for communicating an audio signal from a transmitter unit to a remote receiver unit. The method includes steps of sampling an audio signal into a PCM data stream, modulating the PCM data stream onto a carrier, transmitting the carrier, receiving the carrier, demodulating the PCM data stream from the carrier, and generating a reconstructed audio signal from the PCM data stream. The PCM data stream may be encoded with linear, xcexc-law, or A-law quantization levels. In one embodiment of the method, the quantization scheme is selectable by the user, allowing the user to switch between logarithmically spaced quantization levels (xcexc-law or A-law) best suited for human voice, and evenly-space quantization levels (linear) that may provide better service for some modem signals. The carrier may be an RF carrier using amplitude-shift keying, frequency-shift keying, phase-shift keying, combinations of these, or other modulation schemes to convey the PCM data stream. Alternatively, the carrier may be an IR or visible-light signal transmitted through free space or through an optical fiber. Modulation schemes for the optical carrier include on-off keying (OOK), amplitude-shift keying, frequency-shift keying, and phase-shift keying, among others.
Further, this disclosure presents a transceiver with a PCM coder, an RF transmitter, an RF receiver, and a PCM decoder. The PCM coder receives a transmit audio signal and samples it to generate a PCM data stream. The RF transmitter modulates an RF carrier with the PCM data stream to generate an RF transmit signal. RF transmitter also transmits the RF transmit signal to a remote unit. The RF receiver receives an RF signal from a remote unit, and demodulates the RF received signal to extract a received PCM data stream. The PCM decoder receives the received PCM data stream and decodes it into a received audio signal.
The transceiver may be embodied in a cordless telephone handset, in which case also includes a microphone and a speaker that convert the audio signals to and from acoustic waves. The transceiver may also have a modem port that directly sends and receives the audio signals to and from a modem.